arXiv subjects

Vahid Hatamipour

Publications and source records attributed to Vahid Hatamipour.

4 recordsLinked to original sources

arXiv2022

Revisiting the Figure of Merit of Concentrated Solar Power Receivers

The figure of merit (FOM) is a widely used metric to characterize the performance of concentrated solar power (CSP) receivers by comparing the amount of solar thermal energy retained by the receiver to the incident concentrated solar radiation. However, the FOM is a strong function of the concentration factor and receiver temperature, thus direct comparison of FOM values measured under disparate operating conditions is inappropriate. To remedy this problem, the present study proposes a new metric called the receiver effectiveness calculated by normalizing the actual FOM with its theoretical maximum. The receiver effectiveness can be employed for comparing receiver performances regardless of their operating conditions, and can be treated as more-like the second law efficiency of thermodynamics. In addition, a theoretical limit of the CSP plant efficiency is also examined by combining the maximum FOM and the Carnot efficiency for different concentration factors and receiver temperatures. The calculated maximum CSP plant efficiency clearly indicate that optimizing FOM does not always lead to a better CSP plant performance. Along with the FOM, the proposed receiver effectiveness and maximum CSP system efficiency should be considered as complementary metrics to evaluate the performance of the CSP system.

physics.app-ph↗

arXiv2020

Spatial correlation of the thermally generated electromagnetic field in layered media

A general formulation for the cross-spectral density tensor enabling calculation of the spatial correlation of the thermally generated electromagnetic field in layered media is derived. The formulation is based on fluctuational electrodynamics, and is thus applicable in the near and far field of heat sources. The resulting cross-spectral density tensor is written in terms of a single integration over the parallel wavevector, as the angular integrations leading to numerical instability are evaluated analytically. Using this formulation, the spatial correlation length in the near field of a film made of silicon carbide (SiC) supporting surface phonon-polaritons (SPhPs) in the infrared is analyzed. It is shown that the spatial correlation length of a SiC heat source suspended in vacuum decreases substantially by decreasing its thickness owing to SPhP coupling. In the limit of a 10-nm-thick SiC film, the spatial correlation length is similar to that of a blackbody. The results also reveal that it is possible to control the spatial coherence of a thin SiC heat source via dielectric and metallic substrates, respectively allowing and preventing SPhP coupling. This suggests that active modulation of thermal emission via thin films supporting surface polaritons in the infrared is possible by using a phase change material substrate such as vanadium dioxide.

cond-mat.mes-hall↗

arXiv2019

Spectral redshift of the thermal near field scattered by a probe

The physics underlying spectral redshift of thermally generated surface phonon-polaritons (SPhPs) observed in near-field thermal spectroscopy is investigated. Numerically exact fluctuational electrodynamics simulations of the thermal near field emitted by a silicon carbide surface scattered in the far zone by an intrinsic silicon probe show that SPhP resonance redshift is a physical phenomenon. A maximum SPhP redshift of 19 cm-1 is predicted for a 200-nm-diameter hemispherical probing tip and a vacuum gap of 10 nm. Resonance redshift is mediated by electromagnetic gap modes excited in the vacuum gap separating the probe and the surface when the probing tip is much larger than the gap size. The impact of gap modes on the scattered field can be mitigated with a probing tip size approximately equal to or smaller than the vacuum gap. However, sharp probing tips induce important spectral broadening of the scattered field. It is also demonstrated that a dipole approximation with multiple reflections cannot be used for explaining the physics and predicting the amount of redshift in near-field thermal spectroscopy. This work shows that the scattered field in the far zone is a combination of the thermal near field emitted by the surface, and electromagnetic interactions between the probe and the surface. Spectroscopic analysis of near-field thermal emission thus requires a numerically exact fluctuational electrodynamics framework for modeling probe-surface interactions.

physics.app-ph↗

arXiv2018

Apparent spectral shift of thermally generated surface phonon-polariton resonance mediated by a non-resonant film

The physical origin of spectral shift of thermally generated surface phonon-polariton (SPhP) resonance of a silicon carbide (SiC) bulk mediated by a non-resonant film is elucidated. The local density of electromagnetic states (LDOS) in a non-resonant intrinsic silicon (Si) film due to thermal emission by SiC, derived using fluctuational electrodynamics, exhibits a local maximum near SPhP resonant frequency in addition to a lower frequency resonance generated by gap modes emerging in the vacuum gap separating the SiC and Si layers. Multiple reflections within the vacuum gap also induce a LDOS drop around SPhP resonant frequency. As a result, depending on the film thickness to vacuum gap ratio and the location where the LDOS is calculated in the film, the low-frequency resonance can dominate the LDOS, such that SPhP resonance appears to be redshifted. A similar spectral behavior is observed on the monochromatic radiative heat flux absorbed by the Si film. It is shown that apparent spectral (red and blue) shift of SPhP resonance mediated by a non-resonant film is bounded by the transverse and longitudinal optical phonon frequencies of SiC. This work is of importance in applications involving dissimilar materials, such as thermophotovoltaics and thermal rectification, where gap modes may significantly disrupt flux resonance. Gap modes may also be at the origin of the resonance redshift systematically observed in near-field thermal spectroscopy.

physics.optics↗